ESTRO 37 Abstract book

ESTRO 37

S464

within 1.5%

The Exradin W1 was in agreement with Ω SE

(k max =1.015), which it is consistent with other previously published works, reporting correction factors close to unity for the scintillator in very small fields . The OFs measured with the Exradin A16 and A26 are lower than the Ω SE due to volume averaging effect. The effect is particularly evident for the A16 chamber. The microLion isooctane-filled ionization chamber was in agreement with Ω SE within 1.5%, (k max =0.985), showing a good balance between volume averaging and density effects. The IBA RAZOR and the PTW microDiamond showed an overestimation for the smaller fields, mainly related to the higher density of the sensitive volume. The overall correction factors are reported in Fig.1 and Fig. 2.

Figure 2. A screenshot of the oscilloscope showing implementation of the camera triggering technique.

Poster: Physics track: Dose measurement and dose calculation

PO-0879 Corrections factors determination for small stereotactic radiosurgery beams with a MonteCarlo method A. Girardi 1 , C. Fiandra 2 , F.R. Giglioli 3 , E. Gallio 3 , O. Hammad Ali 2 , R. Ragona 2 1 Universitair Ziekenhuis Brussel, Department of Radiotherapy- Vrije Universiteit Brussel, Brussels, Belgium 2 University of Torino, Department of Oncology- Radiation Oncology Unit, Turin, Italy 3 Azienda Ospedaliero-Universitaria Città della Salute e della Scienza, Medical Physics Unit, Turin, Italy Purpose or Objective The aim of this study was to determine small field output factors (OFs) for an Elekta Axesse linac equipped with circular cones using several active detectors, one passive dosimeter (Gafchromic EBT3 films) and a MonteCarlo (MC) method. The second goal was the determination of correction factors for each active detector using as a reference for each field size a semi-empirical Output Factor value (Ω SE ) derived from the mean of the experimental measurements performed with films and MC dose-to-water calculations. Material and Methods Small fields beams, ranging from 5 to 30 mm in diameter, were defined using circular cones. OFs measurements were performed with six active detectors (Exradin A16, Exradin A26, PTW microLion, PTW microDiamond, Exradin W1 and IBA RAZOR), one passive dosimeter (Gafchromic EBT3) and a MC simulation. The linac treatment head was modelled with the GamBet software, which simulates the transport of energetic particles through matter over the energy range from 50 eV to 1 GeV using the Penelope radiation-physics package. Monte Carlo tasks, such as generation of atomic cross sections, prediction of single-particle interactions with matter and the creation of secondary particles. were performed. The 30 mm cone was taken as machine-specific reference field for the OFs determination. Results A good agreement was observed (better than 2%) between EBT3 films and MC simulation for each field size.

Conclusion Our study showed that it is essential to select an appropriate detector and to apply correction factors if it is needed in order to perform an accurate beam model in small SRS field geometry. Since the PTW microLion and the W1 scintillator were in excellent agreement with the MC simulation and the EBT3 films, they could be used for small fields dosimetry without correction factors. The correction factors should be instead employed for other detectors, in particular for air-filled ionization chamber and solid-state detector in field size smaller than 10 mm. The results furthermore demonstrated that effects such as volume averaging and perturbation effects due to the presence of material with density different to that of water should be taken into account in order to avoid errors in the dose determination and in the machine commissioning.

PO-0880 Independent Evaluation Of Dose Histogram (Dvh) Calculation Accuracy

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